Resonant Conversion of Photon Modes Due to Vacuum Polarization in a Magnetized Plasma: Implications for X‐Ray Emission from Magnetars

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Abstract
It is known that vacuum polarization can modify the photon propagation modes in the atmospheric plasma of a strongly magnetized neutron star. A resonance occurs when the effect of vacuum polarization on the photon modes balances that of the plasma. We show that a photon (with energy E a few keV) propagating outward in the atmosphere can convert from one polarization mode into another as it traverses the resonant density, ρres Yη-2(B/1014G)2(E/1keV)2 g cm-3, where Ye is the electron fraction and η ~ 1 is a slowly varying function of the magnetic field B. The physics of this mode conversion is analogous to the Mikheyev-Smirnov-Wolfenstein mechanism for neutrino oscillation. Because the two photon modes have vastly different opacities in the atmosphere, this vacuum-induced mode conversion can significantly affect radiative transport and surface emission from strongly magnetized neutron stars.